Remove dependency on shared_preferences_ios

[stuartmorgan]

The shared_preferences_ios package has been discontinued, and replaced with shared_preferences_foundation. It looks like the direct dependency on the implementation package is to do background isolate plugin registration. Doing that manually is a legacy approach; ideally you should:

• replace the manual registration code you have now with DartPluginRegistrant.ensureInitialized(), which will register the Dart code for all plugins.
• set your minimum supported Flutter version to 3.0 (where that API was introduced).
• removed the shared_preferences_ios/_android dependencies entirely.

That will avoid this package being bound to implementation details of shared_preferences.

[stuartmorgan]

This is likely related to #130